"The metaphase-anaphase transition in PtK1 cells is delayed by one or more monooriented chromosomes ". Through extensive studies utilizing time-lapse video light microscopy and extensive in vivo laser microbeam surgery, the nature and location of a checkpoint signal was determined. Combined with correlative LM/EM studies , we were able to "demonstrate that molecules in or near the unattached kinetochore of a monooriented chromosome produce an inhibitor of the metaphase-anaphase transition"